In the aircraft industry, it has been generally recognized that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, lithium additions have been made. However, the addition of lithium to aluminum alloys is not without problems. For example, the addition of lithium to aluminum alloys often results in a decrease in ductility and fracture toughness. Where the use is in aircraft parts, it is imperative that the lithium containing alloy have both improved ductility and fracture toughness and strength properties.
With respect to conventional alloys, both high strength and high fracture toughness appear to be quite difficult to obtain when viewed in light of conventional alloys such as AA (Aluminum Association) 2024-T3X and 7050-TX normally used in aircraft applications. For example, a paper by J. T. Staley entitled "Microstructure and Toughness of High-Strength Aluminum Alloys," Properties Related to Fracture Toughness, ASTM STP605, American Society for Testing and Materials, 1976, pp. 71-103, shows generally that for AA2024 sheet, toughness decreases as strength increases. Also, in the same paper, it will be observed that the same is true of AA7050 plate. More desirable alloys would permit increased strength with only minimal or no decrease in toughness or would permit processing steps wherein the toughness was controlled as the strength was increased in order to provide a more desirable combination of strength and toughness. Additionally, in more desirable alloys, the combination of strength and toughness would be attainable in an aluminum-lithium alloy having density reductions in the order of 5 to 15%. Such alloys find widespread use in the aerospace industry where low weight and high strength and toughness translate to high fuel savings. Thus, it will be appreciated that obtaining qualities such as high strength at little or no sacrifice in toughness, or where toughness can be controlled as the strength is increased would result in a remarkably unique aluminum-lithium alloy product.
It is known that the addition of lithium to aluminum alloys reduces their density and increases their elastic moduli producing significant improvements in specific stiffnesses. Furthermore, the rapid increase in solid solubility of lithium in aluminum over the temperature range of 0.degree. to 500.degree. C. results in an alloy system which is amenable to precipitation hardening to achieve strength levels comparable with some of the existing commercially produced aluminum alloys. However, the demonstratable advantages of lithium containing alloys have been offset by other disadvantages such as limited fracture toughness and ductility, delamination problems or poor stress corrosion cracking resistance etc.
Thus only four lithium containing alloys have achieved significant usage in the aerospace field. These are two American alloys, X2020 and 2090, a British alloy 8090 and a Russian alloy 01420.
An American alloy, X2020, having a composition of Al-4.5Cu-1.1Li-0.5Mn-0.2Cd (all figures relating to a composition now and hereinafter in wt. %) was registered in 1957. The reduction in density associated with the 1.1% lithium addition to X2020 was 3% and although the alloy developed very high strengths, it also possessed very low levels of fracture toughness, making its efficient use at high stresses inadvisable. Further ductility related problems were also discovered during forming operations. Eventually, this alloy has been formally withdrawn since 1974.
Another American alloy, 2090, having a composition of Al--2.4 to 3.0 Cu--1.9 to 2.6 Li--0.08 to 0.15 Zr, was registered at Aluminum Association in 1984. Although this alloy developed high strengths, it also possessed poor fracture toughness and poor short transverse ductility associated with delamination problems and prevented alloy 2090 from wide range commercial implementation.
A British alloy, 8090 having a composition of Al--1.0 to 1.6 Cu--0.6 to 1.3 Mg--2.2 to 2.7 Li--0.04 to 0.16 Zr, was registered at Aluminum Association in 1988. The reduction in density associated with 2.2 to 2.7 wt. Li was significant. However, its limited strength capability with poor fracture toughness and poor stress corrosion cracking resistance prevented alloy 8090 from becoming a widely accepted alloy for aerospace and aircraft applications.
A Russian alloy, 01420, containing Al--4 to 7 Mg--1.5 to 2.6 Li--0.2 to 1.0 Mn--0.05 to 0.3 Zr (either or both of Mn and Zr being present), was described in U.K. Pat. No. 1,172,736 by Fridlyander et al. The Russian alloy 01420 possesses specific moduli better than those of conventional alloys, but its specific strength levels are only comparable with the commonly used 2000 series aluminum alloys so that weight savings can only be achieved in stiffness critical applications.
It is also known that the inclusion of magnesium with lithium in an aluminum alloy may impart high strength and low density to the alloy, but these elements are not of themselves sufficient to produce high strength without other secondary elements. Secondary elements such as copper and zinc provide improved precipitation hardening response; zirconium provides grain size control, and elements such as silicon and transition metal elements provide thermal stability at intermediate temperatures up to 200.degree. C. However, combining these elements in aluminum alloys has been difficult because of the reactive nature in liquid aluminum which encourages the formation of coarse, complex intermetallic phases during conventional casting.
Therefore, considerable effort has been directed to producing low density aluminum based alloys capable of being formed into structural components for the aircraft and aerospace industries. The alloys provided by the present invention are believed to meet this need of the art.